drop procedure test;
create procedure test (
  i1 integer,
  i2 integer,
  i3 integer,
  ic set (integer not null)
  ) returning integer as irc;

  define i          integer;

  select count(*) into i from table(ic);

  return i;
end procedure;

update statistics for procedure test;

execute procedure test( 1 , 2 , 3 ,set{ 4 , 5 , 6 });

drop procedure test;

Successfully connected to dbtest as user informix.
HY000:- 674 :[Informix][Informix ODBC Driver][Informix]Routine (test) can not be resolved.
ISAM error:- 111 
Procedure successfully created.
Statistics successfully updated.
 42000 :- 201 :[Informix][Informix ODBC Driver][Informix]A syntax error has occurred.
Procedure successfully dropped.

Successfully connected to dbtest as user informix.
HY000:- 674 :[Informix][Informix ODBC Driver][Informix]Unspecified System Error =  - 674 . sqlerrm(test)
Procedure successfully created.
Statistics successfully updated.
irc
-----------
 3 
 1  rows selected.
Procedure successfully dropped.